Reinforcement fibres of high tensile strength, eg greater than 0.5 GPa, eg about 2 GPa, and high Young's modulus, eg greater than 50 GPa eg about 200 GPa, are widely used in advanced structural materials for a variety of engineering applications. Such fibres which may be of carbon or glass, for example typically have a diameter less than 100 .mu.m, eg typically 5 to 10 .mu.m, to give the high modulus and tensile strength properties. They are set inside a matrix of ceramic, glass, metal or organic polymeric material to provide a structural material having overall improved mechanical properties, eg modulus and tensile strength.
Most high strength, high modulus reinforcement fibres have a roughly circular cross-sectional shape although a `dog-bone` shape is also known.
These known shapes do not give ideal packing properties when such fibres are being set to form a high strength, high modulus composite material and are not in certain cases ideal for fibre manufacture. For example, carbon fibres are made by oxidation of precursor fibres involving diffusion processes and such processes are not ideal with the shape mentioned.
It is an object of the present invention to improve the packing density of high strength, high modulus reinforcement fibres and to reduce the manufacture time of the fibres where this involves diffusion processes, eg as in the case of carbon fibres.